The present invention generally relates to liquid jet recording heads, and more particularly to a liquid jet recording head for carrying out gradation printing on a recording sheet in an ink jet printer.
Recently, much attention has been attracted to non-impact type recording methods because they have an advantage that noise during a printing operation can be reduced to a negligible degree. Among those methods, the so-called ink jet recording method is very helpful because this method makes it possible to perform high-speed recording with no need of using a special fixing process for ordinary recording sheets. Several ink jet recording methods which meet such an objective have been proposed up to now. Some of them have been put into practical use successfully as additional improvements have been made in the proposed methods, but others are still under development.
In the ink jet recording methods, recording is carried out by discharging a stream of droplets of a writing liquid or the ink, to deposit them on the recording sheet. These conventional methods are classified into four major categories according to the technique for generating a stream of ink droplets and the way to control the direction of the moving ink droplets generated.
The first category is the method (Teletype) disclosed, for example, in the specification of U.S. Pat. No. 3,060,429. This method electrostatically generates ink droplets and controls an electrical field applied to the droplets in accordance with recording signals, thus depositing selectively the writing liquid droplets on the recording sheet. More specifically, ink particles are accelerated in an electric field applied between an electrode and a discharge nozzle. Uniformly charged small ink droplets emerge from the nozzle, move across x-y deflection electrodes, and are deposited on the recording sheet while an electric field intensity is controlled, thus forming characters or symbols on the recording sheet. The deflection electrodes are provided to electrically control the movement of the uniformly charged droplet according to the recording signals.
The second method (Sweet) is disclosed, for example, in the U.S. Pat. Nos. 3,596,275 and 3,298,030. In this method, a continuous oscillation is used to generate ink droplets charged at a controlled level. Recording is carried out by shooting the charged ink droplets across the deflection electrodes where a uniform electric field is applied. More specifically, an orifice of a nozzle provided in a recording head provided with a piezoelectric element is disposed at a prescribed distance from the electrodes where recording signals are applied. The piezoelectric element is mechanically oscillated by applying electric signals at a given frequency to the element, allowing the ink droplets to be discharged from the orifice. The discharged ink droplets at the same time are charged by electrostatic induction to have an electric charge in accordance with the recording signals. The charged ink droplets are shot in a deflection direction when moving across the deflection electrodes between which a predetermined intensity of electric field is applied uniformly, so that only the ink droplets carrying the recording signals are deposited on the recording sheet.
The third method (Hertz) which is disclosed, for example, in the U.S. Pat. No. 3,416,153, carries out recording by generating vaporized ink particles through continuous oscillations due to an electric field applied between an annular charging electrode and a nozzle. The condition of vaporization of ink particles is controlled by intensity modulation of the electric field applied between the nozzle and the electrode in accordance with recording signals, thus accomplishing gradation recording.
Finally, the fourth method (Stemme) is disclosed, for example, in the U.S. Pat. No. 3,747,120. The principle of this method is different from those of the preceding three methods. The preceding three methods electrically control the movement of ink droplets discharged from a nozzle and selectively deposit the ink droplets carrying recording signals on the recording sheet. In comparison, the fourth method discharges writing liquid droplets from an orifice of a nozzle in accordance with recording signals to carry out the recording. More specifically, in this method, electrical recording signals are applied to a piezoelectric element provided in a recording head having a nozzle orifice through which writing liquid is discharged. The electrical signals are converted into mechanical vibrations of the piezoelectric element, and in accordance with the mechanical vibrations the record liquid droplets are discharged from the orifice to deposit them on the recording sheet.
While these four conventional methods each have some advantages, there are several unresolved problems they have.
The preceding three methods generate ink droplets directly by electrical energy and use an electric field control technique to control ink droplet deflection. Because of this, the first method provides a simple structure of the device but requires a very high voltage to generate ink droplets as well as has a difficulty in constructing a multi-nozzle recording head and is not helpful in performing high-speed recording.
The second method makes it possible to produce a multi-nozzle recording head and is useful for high-speed recording. However, it requires a complicated structure of the device and is highly difficult to electrically control the movement of ink droplets and often exhibits satellite dots on recording sheet.
The third method has an advantage that image recording with good gradation can be carried out with vaporized ink particles. However, it has a difficulty in controlling the condition of vaporized ink, often exhibits a problem with the recorded image, and is hard to produce a multi-nozzle recording head. This method therefore is not suitable for high-speed recording.
The fourth method provides relatively many advantages when compared to the preceding three methods. Major advantages of this method include a simple structure of the device, no need to recollect ink droplets being not used for image recording, and no necessity to use electrically conductive material as a writing liquid. Unlike the preceding three methods, the fourth method need not recollect ink droplets which are not used for image recording because it carries out the on-demand recording by discharging writing liquid from the nozzle only when a demand is presented. And the fourth method, unlike the first and second methods, has no necessity to select a conductive writing liquid and provides a flexibility in selecting a writing liquid material. However, the fourth method has a problem in manufacturing a recording head, and is very difficult to produce a compact piezoelectric element which has a desired resonant frequency. Therefore, it is hard to produce a multi-nozzle recording head of this type. Further, the fourth method is not suitable for high-speed because it utilizes mechanical vibration the piezoelectric element when ink droplets are discharged.
In view of the device structure, the high speed recording, the multi-nozzle construction, the satellite dot occurrence, the problem with image recording, etc., these conventional techniques can be adapted only to a limited range of the art in which the technique has advantages.
Some attempts for eliminating the above-described problems have been made in several improved recording techniques, which is disclosed, for example, in the Japanese published patent application no. 56-9429 by the same applicant. In this improved technique, the ink in the liquid chamber is heated to increase internal pressure of the ink, allowing the ink to be discharged from a capillary tube nozzle for recording on paper. There are several subsequent proposals for improvements to be made concerning the ink-jet recording technique.
Another technique is proposed, for example, in the Japanese published patent application no. 63-17624. According to the proposed technique, a plurality of heating elements are provided along a line in the writing liquid flow direction within the writing liquid passage leading to a single orifice. With such an improved structure, the proposed technique has a feature that the fartherest heating element from the orifice is first driven. This allows a saving in thermal energy for ink discharging and recording, a high speed recording capability and a long life of the device.
Another technique is disclosed, for example, in the Japanese published patent application no. 62-48585. According to the proposed technique, a plurality of electricity/heat conversion elements are provided in a liquid passage, wherein variable control is carried out to control the timings of drive signals being inputted to the control mechanism to control driving of the conversion elements, thereby performing graduation recording. Further, still another technique is proposed, for example, in the Japanese published patent application nos. 62-46358 and 62-46359. According to the former proposed technique, graduation image recording is carried out by driving a desired number of heating elements, selectively from among a plurality of heating elements provided at positions where writing liquid refilled to a liquid chamber is heated, in accordance with the level of signals carrying image information to be recorded. A conventional recording head which comprises a plurality of heating elements provided on a surface along a liquid discharge path leading to an orifice is proposed. However, since the relationship in location between the orifice and the heating element is not freely selectable, there are only a small number of variations of recording gradation available. According to the latter prior technique, a diameter of a discharge liquid droplet is varied by driving a single heating element, selectively from among a plurality of heating elements with different heating capacities provided at positions where writing liquid refilled to a liquid chamber is heated, in accordance with the level of signals carrying image information to be recorded.
Except for the Japanese published patent application no. 63-17624, graduation recording is a common objective of all the above-described prior art. However, all relate to a conventional type recording head which comprises a heating element provided along a liquid discharge path leading to an orifice. A simple structure of this type recording head having a single heating element corresponding to a single orifice can be formed into a small, highly integrated device and the characteristics of this type recording head are attainable. However, a multiple recording head of the above-described type which comprises a plurality of heating elements corresponding to a single orifice provided along a writing liquid discharge path leading to the orifice is difficult to be formed into a compact, highly integrated structure because a large number of control electrodes are required for controlling independently the heating elements, and therefore the original advantages of the recording head are not easily attainable. Also, in the case of such a recording head having several heating elements, only one or two heating elements which are provided at an appropriate distance from an orifice can serve to effectively generate energy for discharging ink droplets because only a small difference in distance between the heating element and the orifice has a great influence over the liquid discharge speed. The recording head of the conventional type therefore is not necessarily suitable for practical use. Further, in the case of such a recording head, the energy due to bubbles generated in the ink does not necessarily serve to effectively discharge the ink droplets.
Meanwhile, there is a still another gradation recording technique which is disclosed, for example, in the Japanese unexamined patent publication nos. 59-124863 and 59-124864. In this method, gradation recording is carried out by generating bubbles to discharge the writing liquid and by additionally generating bubbles for adjusting the ink discharge energy or by providing small openings at the discharge energy adjusting portion to vary the size of ink droplets produced. However, there is a problem in that it is difficult to match the bubble pressure generated by the main heating element with that generated by the adjusting heating element, because the main heating element is provided at a distance from the adjusting heating element for adjustment of the ink discharge energy. In addition, it is difficult to drive the adjusting heating element under stable operating conditions because the adjustment heating element is located at a closed place and fresh ink is not readily refilled to that place, thus resulting in a rise of the ink temperature there. Therefore, the proposed technique also does not necessarily achieve a desired gradation recording.
Further, another technique is disclosed, for example, in the Japanese unexamined patent publication no. 59-124865. According to the proposed technique, a plurality of ink droplet discharging members are provided for a single orifice, at least one of the members serving as an auxiliary member which provides reliability of the device. However, there is no concept of gradation recording in the disclosure.
As described above, none of the conventional techniques does not attain a desirable result of gradation recording, though several proposed methods are presented for providing a plurality of heating elements or for controlling the timings when the heating elements are driven to carry out gradation recording in a stable manner.